This invention relates to a dielectric insulator separated substrate for semiconductor integrated circuits, which includes a large number of monocrystalline semiconductor island regions in which circuit elements are to be formed and a polycrystalline semiconductor support region for supporting fixedly the island regions.
In a semiconductor integrated circuit, circuit elements such as resistors, diodes, transistors, thyristors and the like may be formed integrally in island regions while being required to be electrically separated from each other. For this, a large number of island regions are electrically insulated from each other and from a support region which supports them. One of the insulating methods uses dielectric materials and a substrate prepared according to this method is called a dielectric insulator separated substrate (hereinafter referred simply to as DI substrate).
The DI substrate, however, undergoes curveness deformations during its preparation process and upon the production of semiconductor integrated circuits. The deformation brings about various defects including cracks in the substrate, degraded accuracy of metal deposition for the electrodes, degradation of the withstand voltage and fluctuations in characteristics of the circuit elements.
The U.S. patent applications Ser. Nos. 604,947 now U.S. Pat. No. 4,017,341 and 637,959 now U.S. Pat. No. 4,079,506 assigned to the same assignee of the present application describe in detail causes for the curveness deformations of the DI substrate and propose a countermeasure therefor. More particularly, there are two types of curveness deformations of the DI substrate. In one type, the DI substrate is deformed convexly toward the side of the monocrystalline semiconductor island regions. In the other type, the DI substrate is deformed convexly toward the side of the polycrystalline semiconductor support region. The curveness deformation of the former type is caused by the difference in thermal expansion coefficient between the monocrystalline island regions and the polycrystalline support region. In the latter type, the deformation results from the wedge action due to oxygen diffused into the polycrystalline support region.
Accordingly, the prior patent applications set forth above proposed to provide a film for preventing the diffusion of oxygen into the surface of the polycrystalline layer of the support region and/or a film for compensating for the difference in thermal expansion coefficient between the island regions and the support region.
According to inventions disclosed in the prior patent applications set forth above, it was confirmed that the DI substrate per se is removed of the curveness deformations but some disadvantages still remain in the production of semiconductor integrated circuits.
Herein, these disadvantages will be explained with reference to the drawing illustrating the production process according to the above prior patent applications.
As illustrated in FIG. 1a, after thermal oxidation of one principal surface of an N-type monocrystalline silicon wafer 1, a mesh (grid) pattern of separation channels 2 is formed on the surface by photoetching. Thereafter, a dielectric insulative silicon oxide film 3 is again formed by heating over the principal surface of the water. Next, as shown in FIG. 1b, a thick polycrystalline silicon layer 4a is formed in a vapor growth reaction furnace, and then a silicon oxide film 5a, a thin polycrystalline silicon layer 4b, a silicon oxide film 5b for preventing the oxygen diffusion and a polycrystalline silicon layer 4c are grown one over another in this order to form a lamination.
The alternate lamination of the polycrystalline silicon layers 4a to 4c and the silicon oxide films 5a and 5b can easily be prepared by introducing at desired number of times water vapor or carbon dioxide into the reaction furnace during the thermal decomposition growth of trichloride silane or quadrachloride silane to thereby react thermally decomposed silicon with oxygen.
By regulating the number of the alternate lamination of the polycrystalline silicon layers 4a to 4c and the silicon oxide films 5a and 5b, the thickness of individual layers and films and the vapor growth temperature, the curveness deformation of the monocrystalline silicon wafer 1 due to the difference in thermal expansion coefficient between the layers and films can be reduced to zero.
By using the flat bottom principal surface of the monocrystalline silicon wafer 1 as datum plane, the polycrystalline silicon layer 4c is polished flatly to a level A designated as a chained line, as shown in FIG. 1b. Next, by utilizing the flattened surface of the polycrystalline silicon layer 4c, the single crystalline silicon wafer 1 is polished flatly to a level B designated as a chained line to thereby obtain a number of monocrystalline silicon island regions 1a, 1b, . . . , 1n insulatedly separated from each other by means of the previously formed silicon oxide film 3.
When forming circuit elements in the plurality of single crystalline silicon island regions 1a, 1b, . . . , 1n by diffusion technique, the extreme outer polycrystalline silicon layer 4c yields the wedge action owing to the heat treatment of the substrate in oxidization atmosphere with the result that the DI substrate as shown in FIG. 1c would undergo the curveness deformation. For this reason, the extreme outer polycrystalline silicon layer 4c was removed by etching and the surface of the silicon oxide film 5b was exposed as shown in FIG. 1d.
As illustrated in these figures, traces corresponding to the separation channels remain on the outer film 5b and the surface of the silicon oxide film 5b becomes considerably irregular.
In addition to the traces corresponding to the separation channels 2, the irregularity results from projections due to local abnormal growth of the polycrystalline silicon.
When diffusing impurities into the individual monocrystalline silicon island regions 1a, 1b, . . . , 1n, the silicon oxide film 5b side of the substrate is attracted by a vacuum chuck and fixed thereto and then a mask is applied to the opposite side of the substrate. Thereafter, a photoresist is applied upon the surfaces of the monocrystalline silicon island regions 1a, 1b, . . . , 1n through the mask. In this process, however, the irregularities present on the surface of the silicon oxide film 5b prevents steady support for the substrate by the vacuum chuck thereby to degrade the masking accuracy.
If the mask was pressed on the surfaces of the monocrsytalline silicon island regions 1a, 1b, . . . , 1n in order to ensure an intimate contact of the mask with the DI substrate, the DI substrate was sometimes broken down owing to the projections which act as fulcrums.